This invention relates to polyethylene films, and more particularly, to polyethylene films which are heat-shrinkable.
Polyethylene films have enjoyed common usage as a packaging material, and especially useful are the heat-shrinkable polyethylene films which can be shrunk to conform to the shape of a wrapped article so as to form a skin-like closure.
In a typical packaging operation, an article to be wrapped is situated within a folded section of film. The folded film is then trimmed and its edges are sealed, usually by the application of a hot wire to form a bag or envelope which thus encloses the article. After the bag or envelope is appropriately vented, it is shrunk tightly around the article by applying heat, such as by passing the package along a conveyor through a hot air tunnel.
To be successfully used in this sort of packaging operation, a shrink film must have a variety of characteristic properties.
One important property is shrink force, as determined by ASTM D1504-70. This is the maximum force exerted in a given direction by a unit cross section of oriented film when heat is being applied. It should be distinguished, however, from an analagous and sometimes greater force measurable during the cooling of a first heated section of film.
In the case of blown or extruded films, shrink force is usually measured in the direction of extrusion, known as the machine direction (MD), and in the direction perpendicular to the direction of extrusion, known as the cross direction (CD).
The MD and CD shrink forces should be sufficient in magnitude to enable the film to fully shrink and conform uniformly to the shape of an article with which it is wrapped. And the magnitudes of the MD and CD shrink forces should not be grossly disproportionate to one another or shrinkage of the film may cause it to wrinkle. Also, when packaging a flexible or non-rigid article, such as a thin pad of writing paper or the like, the MD and CD shrink forces should of course be sufficiently moderate to prevent the article from deforming.
Shrink forces are also a factor for purposes of effective trimming and heat sealing by the hot wire method. If, for example, there is little or no shrink force in a direction perpendicular to the wire, the film will not pull away from the wire as it should, and will melt and cause a build up on the wire of film material. Or, where the shrink force is too high in the direction parallel to the wire, the film along the wire will tend to shrink and bunch up.
A second important shrink film property is minimum shrink temperature, i.e. the minimum temperature to which a wrapped article can be heated to achieve a substantially complete shrinkage of the film in conformity with the article's shape. This minimum shrink temperature is of course desirably as low as possible to conserve on energy requirements for the heat shrinking process.
In some polyethylene films, the minimum shrink temperature has been found to exceed the crystalline melt point of the polyethylene material. Where this is the case, a film will tend to wrinkle and get tacky when heated, and sometimes the wrinkled film will stick to itself, and the wrinkles will not be pulled smooth when shrinkage occurs. Hence, it is most critical for a good shrink film to have a minimum shrink temperature which is below the melt point of the film.
Another important shrink film property is internal tear resistance, as determined by ASTM D1922-67. Films having a high tear resistance will have a propensity to resist tearing, and if they are torn or punctured, the tear or puncture will not be readily propagated or enlarged.
Still another important shrink film property is heat seal energy, as determined by using an Instron Integrator Model No. D1-53 (manufactured by the Instron Corporation). Heat seal energy is basically the energy required to cause failure or separation of a heat seal made by the hot wire method. It is an indication of elongation, as well as the applied force, when failure of the heat seal occurs. Packages wrapped with high heat seal energy films have a greater tendency to stay together when bumped or dropped.
In the prior art, polyethylene shrink films can be found which are quite satisfactory in terms of one or more of the important properties discussed above. It is believed, however, that no previous polyethylene shrink film has been known to be completely satisfactory in respect of all of these properties.
Take, for example, polyethylene shrink films conventionally produced by the basic blown bubble method. These films are made simply by first continuously extruding a tube of the molten film material. The extruded tube is then immediately inflated to form a bubble and thereby stretch the film in the cross direction. This blown bubble is then collapsed between a plurality of rollers which draw the film away from the extrusion station at a rate sufficient to stretch the film in the machine direction. Thus, a film which is produced by this method will typically have some MD orientation, and additional MD orientation can readily be imparted, pretty much as desired, by subsequent in-line or off-line stretching operations which are well known in the art.
A typical polyethylene film conventionally produced by the basic blown bubble method will have good strength in terms of internal tear resistance and heat seal energy. However, the film will have little or no CD orientation and a minimum shrink temperature which exceeds its crystalline melt point.
For a crystalline polymer such as polyethylene to be successfully oriented, it must be stretched under tension at elevated temperatures below the polymer's crystalline melt point. At higher temperatures, the film cannot be oriented and previous orientation tends to be neutralized.
As extruded in the conventional blown bubble process, polyethylene is generally at higher than crystalline melt point temperatures, and thus has insufficient tensile strength for shrink energy to be absorbed and retained. When the extruded tube is inflated, the film is cooled and at the same time undergoes CD stretching. Cooling of the film may be accomplished simply by heat transfer to ambient still air, or devices such as air rings may be employed to facilitate cooling. But in any case, CD stretching of the tube tends to occur at those points where the film is weakest, i.e. the hottest, and is generally completed before the film can cool down to orientation temperatures. This explains why conventionally blown polyethylene films typically have little or no CD orientation.
Still, the blown, MD oriented polyethylene films can be given CD orientation, but the ability to do so is limited by the phenomenon called "cold draw". Simply stated, cold draw is the tendency of a film which has been oriented in one direction, e.g. the machine direction, to stretch locally rather than uniformly in the other direction, e.g. the cross direction. That is to say, elongation tends to commence in one or more isolated areas from which further elongation proceeds; and only when stretched far enough does the entire film length become uniform in orientation and thickness. Cold draw is a phenomenon peculiar to crystalline polymers and occurs on stretching at below crystalline melt point temperatures.
One way to deal with the cold draw problem is simply to fully stretch the film until the desired uniformity in CD orientation and gauge has been achieved. However, where this is done the film will inevitably have high CD shrink forces, which are normally on the order of 250 to 500 psi, although values as low as 100 psi have been reported. And for some applications, high shrink forces such as these are not suitable.
Polyethylene films which have been CD stretched through cold draw can be made by a variety of means well known in the art, such as by using a tenter frame or, for example, by the process disclosed in U.S. Pat. No. 3,231,642 to M. Goldman et al. But these films are usually characterized by low strengths in terms of internal tear resistance and heat seal energy, and by relatively high CD shrink forces. Such films are generally excellent in other respects, however, and have good minimum shrink temperature characteristics. Clysar EH (made by E. I. DuPont de Nemours and Company) is an example of a polyethylene shrink film of this type.
As an alternative to stretching the film through cold draw, T. A. Loredo teaches in his U.S. Patent Application Ser. No. 543,998 filed Jan. 24, 1975, now abandoned, that mild CD orientations can be obtained by substantially restraining the expansion of a conventionally extruded tube while cooling the film to its crystalline melt point. After the film has reached this temperature, the tube is allowed to expand while cooling to a temperature about 50.degree. C. below the crystalline melt point (or points). As the means to restrain tube expansion and effect cooling, a special air ring device is disclosed. Films made by this method have been verified to have mild biaxial orientations and high strength in terms of internal tear resistance and heat seal energy. The minimum shrink temperature of these films, however, typically exceeds the polyethylene crystalline melt point, and the extent to which a CD orientation can be imparted is disclosed as being somewhat limited.
Another way that the cold draw problem is dealt with is to alter the characteristics of the polyethylene by providing a crosslinked molecular structure. This can be achieved, for example, by irradiation methods such as those disclosed in U.S. Pat. No. 3,144,399 to W. C. Rainer et al. Irradiated polyethylene has a higher tensile strength at elevated temperatures, and better lends itself to being stretched past the point where cold draw is a problem. Good clarity can often be elusive with irradiated polyethylene, however, and to remove haze a high degree of stretching at orientation temperatures may be required. Consequently, commercial irradiated films, such as Cryovac D-925 made by W. R. Grace & Co., are typically characterized by comparatively high MD and CD shrink forces. Another disadvantage to irradiated biaxially oriented polyethylene films is that they typically have relatively low strength in terms of internal tear resistance and heat seal energy. They do have good minimum shrink temperature properties, however, in the sense that wrinkling in a hot air shrink tunnel is not a problem.
It was against this background that this invention was made.